1. Technical Field
The present invention relates to a system and method for reading tapes and, in particular, to a method and apparatus for reading down-level linear track tapes with distorted media.
2. Description of Related Art
As magnetic recording tape becomes thinner and recording track widths become narrower, concern for the effects of tape distortion become more important.
In addition to having wider tracks, the older down-level writing is also usually subject to looser geometric =tolerances than is writing for more recent follow-on products that may use the same tape, but have double (or an integral multiple of) the previous track density. When the up-level product has designated tracks together occupying the same space as previously written tracks, the up-level reading of old data may have a higher proportion of track misregistration with respect to track widths and may be sub-standard. This situation is the problem of xe2x80x9cbackwards compatibilityxe2x80x9d and may require special data xe2x80x9crecoveryxe2x80x9d procedures in severe cases. This problem is especially true if the old tracks are strongly curved along the length of the tape due to previous tension or compression pack profiles as a function of wound pack radius.
A tape pack refers to the tape wound onto a reel or spool and its width distortions can be coupled with temperature and/or humidity stresses along with time of exposure stresses. In some longitudinal positions, the new track, closer to one old track edge, may be more optimal than a new track closer to the opposite track edge. If a down-level wider tape track is being read by a narrower new-generation head read element in one of the possible track choices overlapping the wider track, the distortion-caused curvature of the old wide track may result in the chosen new track narrow reader being out of position. In that case, the new reader may pick up off-track adjacent track signal, old written signal, extra noise, or fringing fields that may degrade quality of the desired track signal. This may cause read errors. Even after error correction attempts, a badly mispositioned reader may find the desired readsignal to be unreadable.
The usual response to a tape error is that a retry is then attempted in which the tape motion is stopped, a certain amount of rewind occurs to a region at or before errors occurred, and a reread is done. This pattern is sometimes referred to as a xe2x80x9cfootball,xe2x80x9d because of its appearance when plotted as velocity versus distance. A certain number of retries in place or with minor transverse repositions may occur. The prior art offers xe2x80x9creposition retriesxe2x80x9d as an attempt to avoid the region of tape debris impact or to assist the clean up of previous debris between tape and recording head or to alleviate the impact of momentary tape edge damage.
The present invention concerns multi-track linear tape with specially written servo patterns and servo readers separate from data readers. The pre-written servo patterns are maintained and common to both generations.
The present invention repositions the reading by using one or more large increment whole new-track spacings in a systematic manner in-response to pack level tape distortion, rather than by using minor position error signal (PES) distance adjustments. In the general case, this will involve the reading of both new track choices and selecting appropriate segments of each for optimal track reconstruction on a per channel basis. This reading technique will require the memory storage of first pass individual channel reads or segments of read lengths into a buffer. The optimal reading of different groups of channel reads will have to occur separately on different passes such that total tracks of optimal reads have to be reconstructed from buffer memory or from a combination of previous read memory and current reading. While this reading procedure may be performed for individual isolated read events, a preferred embodiment of the present invention involves selection of segments from alternate track choices over large longitudinal sections of tape.